ELECTRONIC CIGARETTES: The battery-powered devices made of plastic or metal heat a liquid nicotine solution, creating vapor that users inhale. Some models are disposable, and some are designed to be refilled with cartridges or tanks containing what enthusiasts call "e-juice." Some e-cigarettes are made to look like a real cigarette with a tiny light on the tip that glows like the real thing.
WHAT'S IN THEM: The ingredients in the liquid used in most e-cigarettes include nicotine, water, glycerol, propylene glycol and flavorings. Propylene glycol is a thick fluid sometimes used in antifreeze but also used as a food ingredient.
SELLING POINTS: Users say e-cigarettes address both the addictive and behavioral aspects of smoking. Smokers get their nicotine without the thousands of chemicals found in regular cigarettes. And they get to hold something shaped like a cigarette, while puffing and exhaling something that looks like smoke without the ash, odor and tar.
THE WORRIES: Scientists haven't finished much research on e-cigarettes, their safety and whether they help smokers quit, and the studies that have been done have been inconclusive. The federal government is pouring millions of dollars into research to supplement independent and company studies looking at the health risks of e-cigarettes and other tobacco products - as well as who uses them and why.
GROWING MARKET: The industry has rocketed from thousands of users in 2006 to several million worldwide, leading to the rise of more than 200 brands. Sales have been estimated to reach nearly $2 billion in 2013.
ARRAY OF FLAVORS: While some e-cigarette makers are limiting offerings to tobacco and menthol flavors, others are selling candy-like flavors like cherry and strawberry - barred for use in regular cigarettes because of the worry that the flavors are used to appeal to children.